HIV-associated nephropathy (HIVAN) is a leading cause of renal failure in Blacks. The main pathologic feature of HIVAN is collapsing glomerulopathy, which is caused by the direct infection of podocytes by HIV-1. Infected podocytes show unique phenotypic changes including dedifferentiation, increased proliferation, and changes in the actin cytoskeleton. We have cloned a host gene named sidekick-1 (sdk-1), a transmembrane protein of the immunoglobulin superfamily, as being highly upregulated in HIV-1 transgenic podocytes, and we believe sdk-1 to have a key role in HIVAN pathogenesis. We have shown that sdk-1 is highly upregulated in glomeruli in both HIV-1 transgenic mice and in human kidney biopsies. Our data also show that sdk-1 and its ortholog sidekick-2 (sdk-2) function as homophilic adhesion molecules in cells such that each sidekick prefers to bind to its own kind, and we have begun to map the domains responsible. Moreover, sdk-1 and sdk-2 were shown by others to function as neuronal guidance molecules, targeting axons to specific synapses;this suggests that sdk-1 and sdk-2 may analogously have a critical role in determining and maintaining glomerular architecture normally and in disease. In this proposal: 1. We will determine: a) the critical domain(s) of sdk-1 and sdk-2 responsible for the specificity of sdk-1 and sdk-2's homophilic interactions;b) whether targeted disruption of this critical domain's function in HIV-1 podocytes in vitro and in vivo can prevent HIV-1 induced phenotypic changes. 2. We will identify phenotypic changes in podocytes induced by sdk-1 expression. We will determine: a) to what extent does targeted overexpression of sdk-1 in podocytes in transgenic mice recapitulate HIVAN pathophysiology;b) how overexpression of sdk-1 in podocytes effects their ability to interact with glomerular basement membrane components;c) if sdk-1 upregulation induced by HIV-1 infection changes podocye-podocyte adhesion in vitro? 3. We will characterize the interaction of sdk-1 with the actin cytoskeleton. We will identify:a) the potential intracellular sdk-1 interacting proteins including alpha actinin-4;b) the effects of sdk-1 expression on organization of actin filaments and microtubules in podocytes and on expression of other actin-associated molecules. 4. We will better characterize expression patterns of sdk-1 in normal kidney, HIVAN and other glomerular diseases and examine differences in sdk-1 and sdk-2 expression between HIVAN susceptible and HIVAN resistant mouse strains. These studies should elucidate the mechanisms by which sdk-1 contributes to podocyte dysfunction in HIVAN.